OSO ACCU A200 200 Litres Stainless Steel NANOPUR Insulation SKU 11009745 ErP B

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Stainless Steel

EVERLAST™ titanium alloy

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NANOPUR Insulation

Class-leading retention

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Dual Connections

Flow + return both sides

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Anti-Clocking

Protects heat pump

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200L Largest

Ø595 × 1,265mm

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Wall Brackets

Included

ℹ️ Why does my heat pump need a buffer tank?

Most air source heat pumps require a minimum flow rate to operate correctly. Without sufficient water volume, the pump can "clock" (short-cycle on and off), reducing efficiency and increasing compressor wear. A buffer tank adds system volume so the heat pump runs in longer, more efficient cycles. The buffer also supplies the heat energy required for defrost cycles and, for heating-cooling systems, stores cooled water during summer operation. The 200L A200 is designed for larger or more demanding systems — particularly useful for heat pumps above 10kW, whole-house multi-zone systems, or commercial-grade residential installations.

The OSO ACCU A200 is the largest model in the OSO ACCU domestic buffer tank range, providing 199 litres (nominal 200L) of system volume in a wider Ø595mm × 1,265mm column. Moving up to the larger diameter versus the A60/A100's 434mm footprint means the A200 can be accommodated in most residential plant rooms while delivering substantially more thermal mass — reducing short-cycling risk even on larger heat pumps running at partial load across a multi-zone system.

Like all ACCU tanks, the A200 is built from EVERLAST™ titanium-alloyed stainless steel with ULTRAWELD™ construction and NANOPUR™ insulation — no enamel lining, no sacrificial anode, no periodic maintenance of protective coatings. Its ErP B rating confirms efficient standby performance (46W heat loss), and the dual flow/return connections on both sides offer the flexibility needed for complex primary/secondary circuit arrangements found in larger systems.

Key Features

Large 200L System VolumeThe largest domestic ACCU buffer, providing 199L of system volume to prevent short-cycling in systems with high heat pump output or many independently controlled zones. Particularly effective on systems with 10kW+ heat pumps.

Hydraulic Coupling & BalancingDecouples the heat pump primary circuit from the distribution circuit — ensuring the heat pump always sees its required minimum flow rate regardless of how many zones are actively calling for heat.

Anti-Clocking Protection200L of additional volume means even during very low demand periods (mild weather, single zone) the heat pump completes long, efficient cycles rather than short-cycling — protecting the compressor and keeping seasonal COP high.

Defrost Heat ReserveProvides abundant thermal mass for the heat pump to complete defrost cycles without drawing energy from the heating circuit — especially important for systems in exposed locations or at high altitude where defrost frequency is higher.

Cool Storage for Heating-Cooling SystemsFor reversible heat pump systems, the A200 stores cooled water during summer cooling operation while the heat pump simultaneously services DHW heating — the larger volume extends the useful cooling window between compressor cycles.

NANOPUR™ Insulation · ErP B · 46W LossesDespite its larger volume, standby heat loss is just 46W — lower than the 100L model's 55W in absolute terms, demonstrating the efficiency benefit of NANOPUR insulation at larger diameter. ErP B compliant.

EVERLAST™ Stainless Steel · ULTRAWELD™Titanium-alloyed stainless steel with OSO's ULTRAWELD welding process — no enamel to crack, no anode to replace, no corrosion. Over 40% lighter than equivalent enamelled buffer tanks. Designed for a full service lifetime without relining.

Dual Flow/Return ConnectionsTwo complete sets of flow and return connections provide maximum flexibility for primary and secondary circuit layouts in complex multi-zone systems. G½" air vent connection and drain point factory-fitted.

Technical Specifications

Specification	Detail
Brand / Range	OSO Hotwater / ACCU
Model	ACCU A200
SKU / Product No.	11009745
Actual Volume	199 litres
Nominal Capacity	200 litres
Energy Rating (ErP)	B
Standby Heat Loss	46W
Tank Material	EVERLAST™ titanium-alloyed stainless steel
Construction	ULTRAWELD™ — no enamel, no anode required
Insulation	NANOPUR™
Diameter (Ø)	595mm
Height (H)	1,265mm
Weight (empty)	39 kg
Freight Volume	0.48 dm³
Placement	Floor-standing (wall-mountable with included brackets)
Flow/Return Connections	Dual sets — flexible configuration
Air Vent Connection	G½" F (separate)
Drain Connection	Yes — factory fitted
Sensor Pocket	1× for 6–8mm sensor
Wall Brackets	Included (corrosion-resistant)
Typical Applications	Air source heat pumps 10kW+, multi-zone systems, ground source, solar thermal, commercial-grade residential

Frequently Asked Questions

When would I choose the 200L over the 60L or 100L buffer tank?

The 200L A200 is appropriate for heat pump systems with outputs of approximately 10kW or more, multi-zone systems where many zones may be closed simultaneously (giving low overall flow rates during mild weather), or where the installer wants additional defrost margin for installations in colder or more exposed locations. As a general rule, 20–50L per kW of heat pump output is a common starting point for buffer sizing, though the exact volume must be calculated based on the specific heat pump's minimum flow rate, the minimum on-cycle time and the distribution system design. Always consult the heat pump manufacturer's documentation or an MCS-accredited designer.

Why is the A200's heat loss (46W) lower than the A100's (55W) despite being bigger?

This is a function of the surface-area-to-volume ratio and the NANOPUR insulation thickness. As a buffer tank gets larger in diameter, proportionally more volume is enclosed within the same insulated surface area — so the larger tank is actually more thermally efficient per litre of stored volume. The A200's larger Ø595mm diameter (versus 434mm for A60/A100) makes it inherently better insulated in relative terms.

Can the A200 be used with a ground source heat pump?

Yes. The OSO ACCU A200 is compatible with any closed heating circuit — air source, ground source, solar thermal or hybrid systems. Its dual connection configuration and stainless steel construction make it compatible with all common system inhibitors and glycol concentrations typically used in ground source systems.

Is this suitable for commercial applications?

The A200 is designed primarily for larger domestic and light commercial installations. For higher-volume commercial buffer requirements, OSO also offers larger ACCU tanks (from 300L upward). If you are unsure which size buffer is appropriate for a commercial project, contact us for system design guidance.

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Heat Pump Buffer Tanks

OSO ACCU A200 200L Stainless Steel Buffer Tank for Heat Pumps